For calls between parties in a mobile network different services have been developed, such as speech, data and video services. For multimedia service calls, the 3GPP (3rd Generation Partnership project) has standardized a multimedia service transported over UDI (Unrestricted Digital Information) bearers in its TS (technical specification) 26.111.
According to the 3GPP TS 26.111 a call set-up for multimedia calls is standardized, however a service change towards another service, e.g. a speech service is not foreseen. According to 3GPP TS 26.111, if the terminating user equipment or the network providing connectivity towards the terminating user equipment do not support the provision of a multimedia service, a multimedia call to be set up is rejected.
Users wishing to temporarily switch between a speech call and a multimedia call would have to terminate the ongoing call with the old service type and redial to set up a new call for the new service type.
In 3GPP release 5, a new functionality has been introduced that is termed SCUDIF (Service Change and UDI Fallback). SCUDIF is intended to provide a more user-friendly CS (circuit switched) multimedia service and is standardized in the 3GPP TS 23.172.
SCUDIF offers the capability to fallback to a single service i.e. to a speech service or a multimedia service at call set-up and to switch between services during the active phase of the call. The fallback feature allows calls being set-up for providing a multimedia service to fallback to a speech service when a transit network, a terminating network or the mobile terminal of the B-party do not support the multimedia service. Thus the SCUDIF feature provides fewer failed calls. The service change feature within SCUDIF allows the end users to switch between a speech service and a multimedia service.
When a user has setup a speech call and wishes to set up a multimedia call later, she or he will fail if the network conditions or the network equipment providing connectivity for the speech call do not provide the multimedia service. Thus a user repeatedly trying to set up a multimedia call may consume a large amount of signalling resources for unsuccessful service change attempts.